pgsql: Remove undocumented restriction against duplicate partition key

Remove undocumented restriction against duplicate partition key columns.

transformPartitionSpec rejected duplicate simple partition columns
(e.g., "PARTITION BY RANGE (x,x)") but paid no attention to expression
columns, resulting in inconsistent behavior. Worse, cases like
"PARTITION BY RANGE (x,(x))") were accepted but would then result in
dump/reload failures, since the expression (x) would get simplified
to a plain column later.

There seems no better reason for this restriction than there was for
the one against duplicate included index columns (cf commit 701fd0bbc),
so let's just remove it.

Back-patch to v10 where this code was added.

Report and patch by Yugo Nagata.

Discussion: https://postgr.es/m/20180712165939.36b12aff.nagata@sraoss.co.jp

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2131d4501f9472ed3714970e45485e5e892615e8

Modified Files
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src/backend/commands/tablecmds.c | 15 ---------------
src/test/regress/expected/create_table.out | 5 -----
src/test/regress/sql/create_table.sql | 5 -----
3 files changed, 25 deletions(-)